For numerous applications, an electromagnetic actuator is embodied by at least one electromagnet that can be supplied with current, and whose pole face has an associated armature that is spaced from it and held in a first switching position by a restoring spring. When the electromagnet is supplied with current, the armature is brought into contact with the pole face of the electromagnet, counter to the force of the restoring spring. When the current to the electromagnet is cut off, the restoring force of the restoring spring returns the armature to the first switching position. The control element to be actuated is connected to the armature, so the armature simultaneously constitutes a component of the control element.
Also known are electromagnetic actuators that have two electromagnets whose pole faces are oriented toward one another, and are spaced from one another, and in which the armature, which is connected to the control element, moves back and forth between the electromagnets, counter to the force of restoring springs, when the electromagnets are correspondingly supplied with current. Actuators of this type are used, for example, to actuate cylinder valves in piston-type internal-combustion engines. In this instance, in the first switching position, the armature is employed in holding the cylinder valve closed with one of the two electromagnets, and after the retaining current has been cut off and the capturing current has been initiated at the other electromagnet, the armature assists in keeping the cylinder valve in the open position. This means that a cylinder valve in, for example, a four-stroke, piston-type internal-combustion engine is actuated as a function of the crankshaft rpm with a very high switching frequency. Because the very short switching times must be precisely maintained, despite the high switching frequency, it is significant that the armature be embodied with a low tendency toward eddy-current formation, so it can release quickly from the pole face when the retaining current is shut off, for example.
Because the armature plate must have soft-magnetic properties, but is connected to a guide pin that must be wear-resistant and, accordingly, should preferably be produced from a heat-treatable steel material, it is not possible to produce the armature, that is, the armature plate and the guide pin, from only a single material when optimal results are required. With this in mind, it has been proposed to connect an armature plate comprising a plurality of sheets to a steel guide pin in order to utilize the known low eddy-current formation of laminated yoke bodies of transformers or electromagnets in such laminated yoke bodies. An associated problem is the high mechanical stress on the armature plate that occurs upon impact with the pole face of an electromagnet.
It is the object of the invention to create an armature that has a low tendency to form eddy currents, and satisfies both the mechanical and electrical requirements.
According to the invention, this object is accomplished by an armature having the features disclosed in claim 1. The frame element permits a secure connection to the guide pin, on the one hand, and, in connection with the employed plate element, permits the armature to withstand high structurally dynamic stress over a lengthy period of time.
In a first embodiment of the invention, the plate element comprises a sintered metallurgic powder containing a soft-magnetic material.
It is advantageous to use a powder that can be sintered, and in which particles comprising soft-magnetic material remain extensively electrically insulated from one another, despite the sintering process. This can be effected by adding powdered components that can be sintered and are electrically non-conductive, but preferably by using powders in which the ferromagnetic or soft-magnetic and/or electrically conductive powder particles are provided with an xe2x80x9cenvelopexe2x80x9d of electrically non-conductive materials that can be sintered. Thus, when the armature plate has been completely sintered, the incidence of eddy currents is lower than with a purely soft-magnetic metal. It is therefore possible to produce a solid armature plate that, if having a corresponding composition of sintering powder, exhibits only a slight tendency toward eddy-current formation during magnetic reversal.
In a second embodiment, the plate element comprises punch-bundled, adjacent sheet-steel strips that are advantageously produced from a thin transformer sheet.
The armature according to the invention, having a sintered plate element, offers yet another manufacturing-related advantage. The raw part, that is, the guide pin with the frame element and the sintered-in plate element, is cooled rapidly from the sintering temperature down to ambient temperature, then reheated to the heat-treatment temperature of the steel material of the guide pin, maintained at this temperature for a specified period of time and then completely cooled. This method capitalizes on the fact that the sintering temperature for the sintering powders considered here is significantly higher than the hardening and tempering temperature for a heat-treatable material, so after the sintering process has been completed, the further heat treatment can practically be effected in a heated state. During the subsequent heat treatment, the manipulation of the temperature in the heating process can take into account the structural changes in the heat-treatable steel material, without negatively influencing the structure of the sintered armature plate. For example, depending on the type of heat-treatable steel material, it can be advantageous to heat the material in stages, and maintain the temperature at a predetermined level during an intermediate temperature stage, in order to allow the material to form its structure properly.
In an advantageous embodiment of the invention, it is provided that the frame element has a sleeve-like center part, which is connected to the guide pin, preferably through hard-soldering, after the plate element has been incorporated into the frame element, whether through sintering or soldering in the case of a punch-bundled plate element. Thus, in addition to providing a highly stable connection between the armature plate and the guide pin, it is also possible to select a soft-magnetic or non-magnetizable, and/or poorly electrically conductive material having appropriate stability and conductivity properties for the frame element with its sleeve-like center part. Therefore, only the xe2x80x9cbarexe2x80x9d guide pin comprises a heat-treatable steel material and thus xe2x80x9chard-magneticxe2x80x9d material. This makes the conditions unfavorable for the formation of eddy currents in the armature plate via the frame element, which would be the case in material-to-material bonding with the guide pin comprising a hard-magnetic material. This attains the desired reduction in the eddy-current formation.